Background: This study aimed to determine the risk factors of infectious diarrhea in patients undergoing chemotherapy or hematopoietic stem cell transplantation for hematological malignancies. Materials and Methods: This was a prospective, observational study. Patients in whom the infectious agent was determined by laboratory examination were considered to have infectious diarrhea. Patients with diarrhea were categorized as infectious or unidentified and compared in terms of demographic data, treatments, risk factors, laboratory findings, and prognosis. Results: A total of 838 patients were hospitalized, among which 105 patients who met the inclusion criteria were included (12.5%). The patients were divided into two groups: 67 (63.8%) with unidentified diarrhea and 38 (36.2%) with infectious diarrhea. There were no differences between these groups in terms of age, sex, types of hematological malignancies, and presence of comorbidities. The most commonly isolated microorganism was Clostridioides difficile (12.4%). The rate of corticosteroid use was higher in the group with infectious diarrhea (39.5%) than in the group with unidentified diarrhea (7.5%) (P <0.001). The rate of granulocyte colony-stimulating factor (GCSF) use was higher in patients with unidentified diarrhea than in patients with infectious diarrhea (67.2% vs. 42.1%, P=0.022). The median duration of diarrhea was 9 (4–10) days in the group with infectious diarrhea and 5 (3–8) days in the group with unidentified diarrhea (P=0.012). According to the multivariate logistic regression model, corticosteroid treatment increased the risk of infectious diarrhea by a 4.75-fold (95% confidence interval [CI], 1.32–17.02) times. Moreover, the duration of diarrhea may result in a 1.15 (95% CI, 1.02–1.31) fold increase in the risk of infectious diarrhea, while GCSF treatment had a 2.84 (1/0.35) (95% CI, 0.12–0.96) fold risk-reducing effect against infectious diarrhea. Conclusion Infectious diarrhea lasts longer than unidentified diarrhea in patients with hematological malignancies. Although corticosteroid use is a risk factor for developing infectious diarrhea, GCSF use has a protective effect.